The present invention relates to a cathode ray tube and more particularly to an electron gun for a cathode ray tube whose electron lens diameter is increased, whose mass production is simplified, and which can be assembled with precision.
In a cathode ray tube, particularly a high luminance cathode ray tube, such as a projection type CRT, a high luminance, high resolution image is formed on a phosphor screen by increasing the electron beam (electric current) impinging against the phosphor screen, increasing the acceleration voltage applied to the final accelerating electrode (anode electrode) and increasing the potential of the focus electrode.
To reduce the focus distortion of the electron beam, an effort has been made to increase the diameter of the final accelerating electrode as much as possible within a range limited by the inner diameter of the neck portion of the tube envelope.
FIG. 14 is a side view showing one example of the structure of a large-diameter electron gun (hereafter referred to as an electron gun) of the type used in a conventional projection type cathode ray tube. FIG. 15 is a side view of the electron gun of FIG. 14 rotated through 90 degrees about the tube axis Z--Z. FIG. 16 illustrates a focus electrode before being assembled into the electron gun, with the upper half above the center axis of the electrode representing a cross section and the lower half representing a side view. In FIG. 14, the electron includes a first grid electrode 1 (control electrode) having a cathode therein, a second grid electrode 2 (accelerating electrode), a third grid electrode 3 (first anode), a fourth grid electrode 4 (focus electrode), a cylindrical portion 4a being provided on the panel side of the fourth grid electrode 4 (hereinafter referred to as a panel side portion), a cylindrical portion 4b being provided on the cathode side of the fourth grid electrode 4 (hereinafter referred to as a cathode side portion), an intermediate portion 4c of the fourth grid electrode 4, an electrode support 41 for the fourth grid electrode 4, a fifth grid electrode 5 (hereinafter referred to as a second anode or simply anode), and a bead glass 6 that insulates and supports these electrodes. Reference numbers 11, 21, 31 and 51 denote electrode supports for the first grid electrode 1, second grid electrode 2, third grid electrode 3, and fifth grid electrode 5, respectively.
In the drawing, the first grid electrode 1 to the fifth grid electrode 5 are cylindrical electrodes aligned along the tube axis, each of which has a single diameter or two or more diameters. With these electrodes aligned, a pair of softened bead glasses 6 are pressed against these electrodes from lateral directions with respect to the tube axis to embed the electrode supports 11, 21, 31, 41, 51 formed on the electrodes into the bead glasses to fix them together.
In this type of electron gun, a large diameter portion 5a of the fifth grid electrode 5 has a larger inner diameter than the inner diameter of the other electrodes and a small diameter portion 5b of the fifth grid electrode 5 has a smaller inner diameter than the inner diameters of the panel side portion 4a of the fourth grid electrode 4. The panel side portion 4a of the fourth grid electrode 4 is accommodated in the large diameter portion 5a of the fifth grid electrode 5, and the electrode support 51 provided at the small diameter portion 5b of the fifth grid electrode 5 and the electrode support 41 provided at the cathode side portion 4b of the fourth grid electrode 4 are embedded in the bead glass 6 to securely hold these electrodes.
The fourth grid electrode 4, as shown in FIG. 16, has the panel side portion 4a, the cathode side portion 4b and the cylindrical intermediate portion 4c connecting portions 4a and 4b, the three being formed integrally into one piece.
The fifth grid electrode 5 and the fourth grid electrode 4 constitute an electron lens (main lens) that focuses and accelerates an electron beam produced by a cathode K accommodated in the first grid electrode 1 and which emits the focused beam toward the phosphor screen.
This type of electron gun, in which the diameter of the panel side portion 4a of the fourth grid electrode 4 is made larger than the diameter of the small diameter portion 5b of the fifth grid electrode 5, has a large effective lens diameter and therefore enables the cathode ray tube to have an excellent resolution.
The electron gun as shown in the drawing reduces the focus distortion by increasing the diameter of the main lens and increasing the length in the tube axial direction of the fourth grid electrode 4. The electrode shown in FIG. 16 allows the accuracy to be improved by connecting the intermediate portion 4c with the panel side portion 4a and the cathode side portion 4b of the fourth grid electrode 4, both of which have an opening end and form the electron lens.
FIG. 17 is a side view of the structure of another electron gun of the type used in a conventional projection type cathode ray tube. Like reference numerals designate the corresponding functional elements which are identical with those of FIG. 14.
In this type of electron gun, the main lens section comprises a fifth grid electrode (cylindrical second node) 5 and fourth grid electrodes (cylindrical focus electrodes) 431, 432 opposed to each other in the tube axial direction. Reference numerals 41a and 41b denote electrode supports for the divided fourth grid electrodes 431, 432.
The literature disclosing the above conventional technology includes Japanese Patent Publication No. 31696/1983, for example.
In the conventional electron guns described above, the electrode support 41 that fixes the focus electrode 4 to the bead glass 6 is secured, as by welding, to the cathode side portion 4b that constitutes the electron lens. Hence, when the electrode support 41 of the focus electrode 4 is embedded in the bead glass 6, the embedding force deforms the cathode side portion 4b of the focus electrode 4. In other words, the circular shape of the opening end of the focus electrode 4 is distorted, rendering the formation of an accurate electron lens impossible.
Reducing the embedding force to alleviate the electrode deformation when securing the electrode, however, tends to result in a reduced supporting force for the electrode and the bead glass.
In an electron gun having the construction shown in FIG. 17, the focus electrode constituting the main lens is shaped like a cylinder with a single large diameter and is long in the tube axial direction. Since the cylindrical shape of such an electrode influences the electron lens, the electrode is required to have a high precision over the entire length and is difficult to manufacture. If the cylindrical electrodes 431 and 432 of the focus electrode are formed as a single electrode, the electrode length further increases in the tube axial direction and is required to have still higher precision over the entire length. Such a long electrode is easily deformed when it is fixed or when it is transported, thus degrading the characteristics of the electron gun and therefore the image to be produced.
Another conventional electron gun is disclosed in Japanese Patent Laid-Open No. 258346/1989. In this electron gun the focus electrode has a plane surface at the cathode side end which is generally perpendicular to the electron beam and has an electron beam passage opening formed therein. For this focus electrode, however, the precision of the inner diameter of the cylindrical electrode is not taken into consideration at all, though the size of the electron beam passage opening is considered.
Japanese Patent Laid-Open No. 231915/1997 describes an electron gun in which the diameter of the focus electrode on the cathode side is made larger than the diameters of the adjacent electrodes to increase the diameter of the electron lens made up of the first anode and the focus electrode. For the focus electrode disclosed in Japanese Patent Laid-Open No. 231915/1997, however, the precision of the cylindrical portion constituting the electron lens, the ease with which the electron gun is assembled, or the precision of the assembled electron gun are not taken onto consideration at all.
The fourth grid electrode 4 has its one end inserted into the second anode and the other end opposed to the first anode. The electron beam passes through the control electrode and the accelerating electrode and is gradually expanded in diameter, after which it is subjected to focusing by the main lens and is focused on the phosphor screen. The electron beam with an expanded diameter enters the electron lens constituted by the focus electrode 4 and the first anode, and thus the electron lens is required to have high precision. To fabricate a high precision electron lens, the cylindrical portion thereof must be made with high precision.